1. Field of the Invention
The present invention relates to a low temperature method for making a photovoltaic material. In particular, the present invention describes a low temperature method for the deposition in vacuo of successive layers of materials required for a photovoltaic device. The present invention uses ion beam assisted processes in which a selected silicon containing precursor film is controllably converted to an amorphous silicon and carbon mixture. Ion beams are used to control the hydrogen content and thereby control the electrical conductivity of the material. The present invention further comprises the addition of a dopant and the deposition of the electrical contacts, both by thermal evaporation.
2. Description of the Prior Art
Photovoltaic materials, such as solar cells, have been made from amorphous hydrogenated mixtures of silicon and carbon. Prior art methods for making photovoltaic materials have involved depositing such amorphous hydrogenated mixtures by plasma activated chemical vapor deposition (PA CVD) from gaseous mixtures of silane and hydrocarbon. In the prior art, electrical doping of the photovoltaic material has been achieved by introducing diborane, phosphine, or arsine vapor into the plasma. Prior art substrate materials have comprised glass or metal.
There are several drawbacks to prior art methods of photovoltaic material fabrication. The gases used are environmentally hazardous and dangerous to handle. The temperature of deposition is typically at least 250.degree. C. This high temperature precludes the use of polymers as substrate materials. Polymers are cheaper and more flexible than the glass and metal used in the prior art.
Prior art methods of photovoltaic material fabrication have also comprised the forming of electrical connections by evaporation of metal film, such as nickel or aluminum. The vacuum evaporation of metal contacts and electrodes is carried out in a separate facility from the PA CVD in prior art methods of fabrication. This use of separate facilities increases the cost of manufacturing photovoltaic materials using the methods of the prior art.